Wireless communication systems, for example cellular telephony or private mobile radio communication systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTSs) and a plurality of subscriber units, often termed mobile stations (MSs). The term mobile station generally includes both hand-portable and vehicular mounted radio units. Radio frequency (RF) transmitters are located in both BTSs and MSs in order to facilitate wireless communication between the communication units.
In the field of this invention, it is known that continuing pressure on the limited radio spectrum available for radio communication systems is focusing attention on the development of spectrally efficient linear modulation schemes. By using spectrally efficient linear modulation schemes, more communication units are able to share the allocated spectrum within a defined geographical coverage area (communication cell). An example of a digital mobile radio system that uses a linear modulation method, such as n/4 digital quaternary phase shift keying (DQPSK), is the TErrestrial Trunked RAdio (TETRA) system, developed by the European Telecommunications Standards Institute (ETSI).
Since the envelopes of these linear modulation schemes fluctuate, intermodulation products can be generated in the non-linear radio frequency power amplifier(s). Specifically in the digital mobile radio (PMR) environment, restrictions on out-of-band emissions are severe (to the order of −60 dBc to −70 dBc relative to the power in adjacent frequency channels). Hence, linear modulation schemes used in this scenario require highly linear transmitters.
The actual level of linearity needed co meet particular out-of-band emission limits, is a function of many parameters, of which the most critical parameters are modulation type and bit rate. Quantum processes within a typical radio frequency (RF) amplifying device are non-linear by nature. Only a straight line may approximate the transfer function of the amplifying device when a small portion of the consumed direct current (DC) power is transformed into radio frequency (RF) power, i.e. as in an idea linear amplifier case. This mode of operation provides a low efficiency of DC to RF power conversion, which is unacceptable for portable units.
The emphasis in portable PMR equipment is to increase battery life. Hence, it is imperative to maximise the operating efficiencies of the amplifiers used. To achieve both linearity and efficiency, so called linearisation techniques are used to improve the linearity performance of the more efficient classes of amplifier, for example class AB, B or C amplifiers. One such linearisation technique, often used in designing linear transmitters, is Cartesian Feedback. This is a ‘closed loop’ negative feedback technique, which sums the baseband feedback signal in its digital ‘I’ and ‘Q’ formats with the corresponding generated ‘I’ and ‘Q’ input signals in the forward path. This ‘closed loop’ I-Q combination is performed prior to amplifying and up converting the input signal to its required output frequency and power level. The linearising of the power amplifier output requires the accurate setting of the phase and amplitude of a feedback signal.
Details of the operation of such a lineariser is described in the paper “Transmitter Linearisation using Cartesian Feedback for Linear TDMA Modulation” by M Johansson and T Mattsson 1991 IEEE.
The lineariser circuit optimises the performance of the transmitter, for example to comply with linearity or output power specifications of the communication system, or to optimise the operating efficiency of the transmitter power amplifier. Operational parameters of the transmitter are adjusted to optimise the transmitter performance and include as an example, one or more of the following: amplifier bias voltage level, input power level, phase shift of the signal around the feedback loop. Such adjustments are performed by say, a microprocessor. Due to the sensitivity of such transmitter circuits, a range of control and adjustment circuits and/or components are needed so that a linear and stable output signal can be achieved under all operating circumstances.
All linearisation techniques require a finite amount of time in which to linearise the performance of a given amplifying device. The ‘linearisation’ of the amplifying device is often achieved by initially applying a training sequence to the lineariser circuit and the amplifying device in order to determine the levels of phase and gain distortion introduced by the linearisation loop and the amplifying device. Once the phase and gain distortion levels have been determined, they can be compensated for, generally by adjusting feedback components/parameters.
To accommodate for such linearisation requirements, communication systems typically allocate specific training periods for individual users to train their transmitters. The TErrestrial Trunked RAdio (TETRA) standard includes a time frame, termed a Common Linearisation Channel (CLCH) as is described in UK Patent Application No. 9222922.8, to provide a full-training period approximately once every second. The CLCH frame allows a radio to ‘train’ prior to gaining access to the system. However, a radio having to wait up to one second before training and then accessing the system is undesirable. To assist this significant delay in call set-up times, and also provide an additional period for fine tuning a radio's output characteristics, due to changes in temperature, supply voltage or frequency of operation, a reduced training sequence has been inserted at the beginning of each TETRA traffic time slot for the radio allocated that slot to perform a minimal amount of training or fine tuning. This period may be used for phase training.
An example of such a training sequence is described in U.S. Pat. No. 5,066,923 of Motorola Inc., which describes a training scheme where the phase of the amplifier is adjusted in an ‘open-loop’ mode and the gain of the amplifier is adjusted when the loop is closed.
During phase training, the Cartesian feedback loop is configured to be ‘open loop’, i.e. a switch is used to prevent the fed-back signal from being combined with the signal routed through the transmitter circuit. In this regard, in a phase training mode of operation, a positive signal is applied to the I-channel input. The phase shift around the loop is measured and, in response to the measured I-channel phase shift, the phase around the loop on both the ‘I’-channel and the ‘Q’-channel is adjusted by a phase shifter.
FIG. 1 illustrates a phase diagram 100 with a perfect I/Q quadrature balance, i.e. a 90-degree phase difference between the I-channel 120 and the Q-channel 110. The Cartesian loop is opened and a positive baseband signal applied to the input of the ‘I’-channel. Phase training control circuitry monitors the signal before switch 426 on the Q channel—indicated as Vfq 455. A successive approximation register (SAR) phase training algorithm controls the phase shifter and minimises the Vfq voltage. At the end of the SAR algorithm, phase training corrects the loop phase by an angle β. A voltage value prior to the switch on the Q channel is then reduced close to zero. The sane process is repeated for a negative baseband signal input to the I-channel. The calculated results from both the positive and negative training applied to the I-channel are averaged and used to adjust the phase around both the I-channel loop and the Q-channel loop.
The inventors of the present invention have recognised and appreciated that, in practice, the perfect I-Q 90-degree relationship is rarely achieved. This imbalance results from the various component tolerances within the respective ‘I’ and ‘Q’ loops. An unbalanced phase relationship 200 is illustrated in FIG. 2. Here, Q′ is the actual loop's quadrature axis 210. The Q′ axis 210 deviates from the ideal Q axis 110 by α degrees 220. Again Vfq is minimnized using Q′ axis 210 as quadrature. From FIG. 2, we can see that instead of correcting the loop phase by β degrees 230, the phase training process has corrected the loop phase by β−α degrees 240. This means that phase training provides a result that is inaccurate by α degrees 220. With reference to FIG. 4, the I/Q imbalance α may be in the forward 436 or feedback 422 I/Q generator. For example, the feedback I/Q generator 422 may shift the LO phase by 90-α degrees, instead of an ideal 90 degrees.
The inventors of the present invention have identified that any quadrature imbalance in the generation of linearised signals during the training sequence of a Cartesian loop transmitter may cause significant phase training errors. Accurate phase training is a critical stage in the linearisation of such transmitter circuits, as the phase accuracy has a substantial effect on loop stability and wideband noise.
Thus, there currently exists a need to provide an improved transmitter circuit, and in particular a mechanism for improving phase training accuracy, wherein the abovementioned disadvantages may be alleviated.